Method and apparatus for minimizing the horizontally-traveling components of seismic waves



C. G. DAHM 2,473,469 METHOD AND APPARATUS FOR MINIMIZING THE HORIZONTALLY-IRAVELING COMPONENTS OF SEISMIC WAVES Filed April 10, 1948 5 Sheets-Sheet 1 we r. R M a p p M M M M 6L r ,L r 1 I 5 4 R7 flfin Q M M, A A r p R i V M M p P M M 5 am G m 5 amp a F 6 F Q M M A A. AIDZIY R 3 v M 1 p P M M a a MG Q P M M A L. Q. w a Q AIDFIY M M m0 m6 v M a .J4 M M 6 P p M M F4 e. 0 k I\ P 4 Mi 2 AM q, a M M 64 P P r Q J & no 2 A 0. Y

BY 2 z 6 ATTO C. G. DAHM PARATUS FOR MINIMIZING THE June 14, 1949.

METHOD AND 'AP HORIZONTALLY-TRAVELING COMPONENTS OF SEISMIC WAVES Filed April 10, 1948 3 Sheets-Sheet 2 5 JV 7 g .3 r

7 m M in: Aw Z INVENTOR BY g ATTO EY iqfk.

June 14, 1949. c. e. DAHM 2,473,469

METHOD AND APPARATUS FOR MINIMIZING THE HORIZONTALLY-TRAVELING COMPONENTS OF SEISMIC WAVES Filed April 10, 1948 3 Sheets-Sheet 3 RECJRDER INVENTOR Cam/00: 61 04/7 E l [1 a ATTORN Y Patented June 14, 1949 METHOD AND APPARATUS FOR G THE HORIZONTALLY-TRAVELING COMI- PONENTS F SEISMIC WAVES Cornelius G. Dahm, Dallas, Tex., assignor, by

mesne assignments, to Socony-Vacuum Oil Company, Incorporated poration of New York New York, N. Y., a cor- Application April 10, 1948, Serial No. 20,233

This invention relates to seismic prospecting, and particularly to geophone spread arrangements suited to obtain valuable seismic information despite unfavorable condition of the subsurface terrain.

In general, in seismic prospecting a charge of explosive is detonated at or below the surface of the earth and the resulting seismic waves, as reflected from subterranean strata, are detected by a spread of geophones, or equivalent detector devices, whose electrical outputs are impressed upon a multi-trace seismograph to produce a record which may be analyzed in determination of the depths and contours of the strata. In areas where hard rock is at or near the earths surface, the seismic records obtained with the usual geophone arrangements yield little or no information of value because of high noise level throughout the frequency range of the reflected energy. It has been determined that a considerable amount of most of this noise energy picked up by the geophones travels horizontally and at quite constant velocit in a given area.

In accordance with the present invention, there are utilized several pairs of geophones for the production of each record trace; the detectors 'of each pair are suited, by their horizontal spacing, by the phasin of their individual outputs and by an associated filtering and mixing means, to provide a joint output in which is predominant the reflected wave energy; horizontally traveling energy the same frequency as that of reflected energy having been eliminated as the spacing of the geophones affects them in phase opposition. The spacing of the geophones is different for the different pairs so that collectively their'outputs cover the entire band of seismic frequencies. By combining the outputs of these pairs of geophones in a single recorder circuit, there is produced a trace of the reflected waves to the substantial exclusion of much stronger horizontally traveling waves.

. Further and preferably in accordance with the invention, the complete record resulting from a single explosion comprises a plurality of traces each produced as above stated: to that end, there are utilized several geophone spreads, each comprising several pairs of equally spaced geophones,

the output of each pair of one spread being combined,as above briefly described, with-th outputs of corresponding pairs of the other spreads, the record containing reflected waves to the exclusion of horizontally traveling waves.

The invention further resides in th methods and systems hereinafter described and claimed.

4 Claims. (Cl. 177-352) For a more detailed understanding of the invention, reference is made to the accompanying drawings in which:

Fig. 1 is a block diagram showing a geophone arrangement and associated circuit components for producing a single record trace of reflected seismic waves;

Fig. 2 is a schematic diagram of a filter network and associated-tubes used in the geophone channels of Figs. 1 and 5;

Figs. 3 and 4 are schematic views of mixer circuits usable in the systems of Figs. 1 and 5; and

Fig. 5 is a simplified block diagram of a geophone arrangement producing a multi-trace record of reflected seismic waves.

As above briefly stated, when hard rock is at or near the earths surface, the records obtained are usually of little value because the reflected waves are completely obscured by noise due to much more intense horizontally traveling waves produced by the same explosion. It has been determined this horizontally traveling energy moves at more or less constant velocity, of the order of about ,6,000 feet per second, and this constancy, as will hereinafter appear, is taken advantage of in elimination or minimization of the disturbing effect of this horizontally traveling energy upon the record of the reflected waves.

By recourse to narrow band-pass filters, it is possible under these adverse circumstances to detect the reflected waves. It is necessary, however, that they pass only a very restricted band within a range in which the reflection-to-ncise ratio is favorable; because the band-width is narrow, the reflections, though they may appear. resemble sine waves in appearance, and do not have such distinctive character that the reflected waves from different strata can be distinguished. This expedient is, therefore, unsatisfactory.

The record to be of value in seismographic exploration must be free of obscurement by'horizontally travelin waves and yet must preserve the identity of the waves reflected from the various strata.

Referring to Fig. 1, the geophone arrangement there shown as exemplary of a simple form of the invention comprises four spreads S1-S4, each comprising at least one pair of geophones. For simplicity of explanation, in discussion of Fig. 1', each spread will be described as though consisting of only one pair of geophones, but as will later appear in discussionof Fig. 5, the relations discussed in connection with each pair of geophones of a spread equally apply to other pairs of geophones of the same spread.

quency are substantially attenuated so that the output of this channel is confined to a narrow. band of frequencies including frequency, F1. The output energy of channel C181 has two components, one corresponding with the reflected wave energy, and the other corresponding with the horizontally traveling wave energy.

Similarly. the amplified channel C281 for the other geophone G2S1-of the pair also includes a narrow band-pass filter C2F1 peaked at f .quency F1. The output of channel C281 therefore comprises only energy having frequencies in the immediate neighborhood of frequency F1; part of this energy is due to the reflected waves, but a.

larger part of it is due to the horizontally traveling waves.

The outputs of the two amplifier channels C181 and C281 of the spread 81 are impressed upon a mixer CMS1. As the geophones G181 and G181 are spaced by one-half wavelength at the frequency F1, the horizontally traveling wave energy actuates them 180 degrees out of phase, with the result that the components of the signals of the two channels C1 S1 and C181, which correspond with the horizontally traveling wave energy, are cancelled out in the mixer CMS1. The reflected wave energies, however, are in phase in the signal outputs of the geophones G181 and G281, so that in the mixer CMS1 the reflected wave components of the signals of the two channels C181 and C281 are additive or in phase.

The horizontal separation between the pair of geophones G182 G182 of the second spread 82 is different from that of the separation between the geophones of spread 81 and correspondswith an odd number of half-wavelengths at another seismic frequency F2. Each of the band-pass filters C1F2, C2F2, respectively included in the geophone channel C182 and C281, are designed'to cut off or attenuate all frequencies somewhat higher and somewhat lower than frequency Fr. Consequently, the individual signal output of channels C182 and C282 include only a band of frequencies in the neighborhood of frequency F2. Some of the output energy of each of the channels corresponds with that of reflected waves and the remainder or major portion corresponds with that of the horizontally traveling waves. Again, however, generally as above discussed in connection with spread 81, the components of the signals due to the horizontally traveling waves cancel out because as introduced into the mixer CMS: they are in phase opposition, whereas the signal components due to the reflected wave energies are there in phase or additive.

The spacing D2 between the pair of geophones G183 and G281 of the third spread 83 is different from that of spreads S1 and S2 and is an odd number of half-wavelengths at another seismic frequency F3. The filters 01F: and CaFz, respectively associated with the geophone channels C18:

and C283, each pass a band of frequencies in the neighborhood of frequency F3. Accordingly, those fractions of the outputs of the geophone channel C183 and C283 due to the horizontally traveling wave energy are in effect cancelled in tially only a narrow band including frequency F1;

some of the energy is due to reflected waves and the remainder or major portion is due to the horizontally traveling waves. Because of the geophone spacing, the signal components due to the horizontally traveling energy are in phase opposition at mixer CMS1 and so are cancelled, whereas, the signals due to the reflected waves, are substantially in phase.

The outputs of the mixers CMS1, CM82, CMS: and CMS1 of the four spreads 81-84 are combined in the inter-spread mixer SMT1. The output of the mixer SMT1 is impressed upon the trace circuit T1. As the output of the interspread mixer includes all of the significant seismic wave frequencies and as all of the energies of the horizontally traveling waves of those same frequencies have been minimized or eliminated, the trace includes the identifying characteristics of the reflected waves and clearly shows them without obscurement by the much more intense horizontally traveling waves.

Each of the band-pass filters C1F1-CzF1 may be of any suitable type having proper circuit constants and elements to obtain the desired narrow band-pass effect; specifically, the filter networks may be of the ladder type or of the type using coupled tuned circuits. The circuit constants, of course, are different for the filters of the different spreads, in each case selectively to pass frequencies corresponding with the geophone spacing of the same spread. The filter network F, Fig. 2, is generically illustrative of a suitable form of filter network for any of the band-pass filters of Figs. 1 and 5. More particularly, the input conductors II and I2 of Fig. 2 extend from the associated geophone or from an amplifier interposed between it and the tube V1. Preferably, and as shown, the tube V1 may be of the pentode type in order that a control voltage may be applied to the suppressor grid, generally as disclosed in U. 8. Patent 2,408,001, to vary the channel gain as a function of time subsequent to the shot or explosion which initiates the reflected waves of interest. The screen grid or #2 grid of the tube V1 is connected to the terminal l3 of a suitable source of direct current voltage, preferably through a filter comprising resistance It and condenser IE to prevent interstage reaction. The anode of tube V1 is connected to the terminal ll of a suitable source of higher direct current through a filter network comprising resistance I8 and condenser [9 to avoid intercircuit reaction. The anode circuit of tube V1 includes the load resistor l6 coupled to the filter F by a condenser 20. The filter F comprises one or more low-pass sections, each comprising series inductances 2I22 and an intermediate shunt guano designed to pass only a narrow band of frequencies whose mean frequency is determined by the spacing between a'pair of geophones of the same spread. The composite of the frequency-selective characteristics of each group of filters (one from each spread) covers substantially the range of seismic reflection frequencies. The output of filter F is impressed onthe control grid of tube V2, the output of which is coupled through the circuit comprising condenser 32 and conductors 34 and 35 to the mixer CMSi.

The inter-channel mixers CMS1.CMS4 may be of any suitable known type: each, for example, may be of the type generically represented by the mixer CMS1 of Fig. 3 including a dual-triode tube V3 such as 608. The lead 42 extends from one of the grids of tube V3 to the output transformer or equivalent of one of the geophone channels, for example, of channel C1 of spread S1, and the lead 43 from another grid of the tube V: extends to a similar output element of the other of paired channels, for example, channel C2 of the same spread S1. The output voltage Es 'r of the mixer, as appearing, for example, across the secondary of its output transformer, is therefore a function of or corresponds with the vector summation of the voltages Ec and E As previously explained, these voltages each include one component corresponding with the reflected waves within the band of frequencies passed by the channel filter and a second component corresponding with the horizontally traveling waves within the same band of frequencies. As the second component of these voltages as applied to the grids of tube Va are substantially equal and 180 out of phase, because of the geophone spacing, they cause little or no change in the anode currents of tube V3 and the output voltage Es 'r of the mixer is substantially free of any component due to the horizontally traveling waves. The reflected wave components of the voltages Ec and E0 on the contrary are substantially in phase and so cause cumulative change in the anode current.

The inter-spread mixer SMT1, Fig. 1, for combining the'outputs of the mixers CMS1CMS4 may also be of any suitable known type: each may be of the type generically represented by the mixer SMTI, of Fig. 3 or of Fig. 4. In the particular four-spread system shown two dual-triodes V4 and V5 may be used for the mixing. In the circuit of Fig. 3, the output voltages of the channel mixers CMSi-CMS4 are respectively impressed upon the grids of the spread-mixer tubes, V4 and V5. In Fig. 4, the output voltages of channel mixers CMS1-CMS4 are mixed in the resistance network which includes resistances 48 connected to recording element 49. The result in both Fig. 3 and Fig. 4 is that the output voltage Er of the spread mixer SMTI, includes all of the seismic wave frequencies and corresponds only with the reflected wave energies picked up by one pair of geophones of each of the spreads. All of the frequencies of the reflected waves are included because of the overlap of the frequencyselective curves of the filters CiFi-C2F4. The disturbing effects of the strong horizontally traveling waves within the same range of frequencies are suppressed or entirelyieliminated by the above described spacing of the geophones of each pair, the filters associated with each pair, and the channel mixers. I

To produce a multi-trace record, each spread, as shown in Fig. 5, includes a plurality'of pairs of geophones or equivalent seismic detectors.

The spacing between each of the several pairs of geophones of the same spread is the same and is different from the spacing between the geophones of the other spreads. Moreover, the

band-pass filters associated with the pairs of geophones of a particular spread are all similar in that they pass the same band of frequencies whose mean frequency'corresponds with the geophone spacing of that spread.

Specifically, the spacing D1 betweenthe pairs of geophones G1S1G1S1 of spread S1 may be 100v feet corresponding with a half-wavelength at a frequency of 30 cycles which is the mean frequency of the band passed by the filters Fs of spread S1; the spacing D2 between each pair of the geophones G1S2G'1S2 of spread S2 may be '75 feet corresponding with a half-wavelength at a frequency of 40 cycles which is the median frequency of the band passed by the filters Fe of spread $2; the spacing D3 between each pair of the geophones GiSs-GrSs of spread S: may be feet corresponding with a half-wavelength at a seismic frequency of 50 cycles which is the median frequency of each of the band-pass filters Fs of spread Sn; and the spacing D4 between each pair of geophones G1S4-G1S4 of spread S4 may be 50 feet corresponding with a half-wavelength at a frequency of 60 cycles which is included in the band passed by filters Fs of spread S4.

The particular system shown in Fig. 5 produces a six-trace record, but it shall be understood the number of geophones in each spread may be increased or decreased to obtain a correspondingly larger or smaller number of concurrent record traces. In this connection, it is pointed out that the number of geophones may be and preferably is less than twice the number of pairs of geophones; as shown in Fig. 5, each of the geophones, except the ones at the ends of the spreads, is connected to two channels. Further, it is to be noted that the geophones G4S1, G4S2, G4S3 and G454 are centrally located in their respective spreads 81-84 and in such an arrangement they may be replaced by one geophone electrically common to all four spreads 81-44. For the particular example cited above for illustration purposes where geophone spacings were 100 feet, 75 feet, 60 feet and 50 feet for the different spreads, G481 would also serve as G4S2, G4S3 and G484; G252 as G154; G331 as (32534; GsSi as G654; and G632 as G784 in which case 21 geophones would be electrically equivalent to the 28 geophones shown in Fig. 5.

It shall also be understood the number of spreads may be increased or decreased though about four, as shown, are preferred.

Though omitted for simplicity in Fig. 5, it shall be understood that with each pair of geophones of each spread there shall be a two-channel mixer and that with the two-channel mixers of the corresponding pairs of geophones of the several spreads there shall be associated an interspread mixer, one for each trace circuit (Tl-T6, Fig. 5). In other words, the arrangement shown in Fig. 1 is in efiefict duplicated for each additional trace to be made. The number of channel filters may be reduced by 'using one filter for each pair of channels: for example, the filters CiFi and CsFi of Fig. 1 may be replaced by a single filter having the same frequency selective characteristic preferably disposed between the'channel mixer CMS1 and the associated Spread mixer SMTi. 4

While a preferred embodiment oi the invention has been described, it will be understood that modifications may be made within the scope of the appended claims.

What is claimed 7 1. In a system for seismic prospecting, an arrangement of seismic wave detectors comprising pairs of detectors, the detectors of each pair being horizontally spaced from one another at distance corresponding with an odd number of half-wavelengths at a predetermined seismicfrequency different for the different pairs of detectors, output circuitsfor each of said pairs respectively including filter means selectively to pass the frequency corresponding with the spacing betweenthe detectors of that pair, and a recorder circuit connected to said output circuits to produce a record trace of reflected seismic waves having said predetermined frequencies and substantially free of horizontally traveling waves having said predetermined frequencies.

2. In a system of seismic prospecting, a plurality of geophone spreads, each spread comprising geophones horizontally spaced at distance corresponding with an odd number of half-wavelengths at apredetermined seismic frequency different for the different spreads, a plurality of output circuits, one for each pair of geophones of each spread and respectively including filter means selectively to pass the frequency corresponding with the spacing between the geophones of that pair, and a plurality of recorder circuits each including the output circuits of a pair of geophones of each spread.

3. In seismic prospecting, the method of minimizing confusion of the record of reflected seismic waves by co-existent horizontally traveling waves which comprises arranging geophone spreads with the horizontal spacing between the geophones of each spread corresponding with an odd number of halt-wavelengths at a predetermined seismic frequency different for the different spreads, combining in phase the reflected wave outputs of each pair of geophones of each spread having the frequency corresponding with their spacing, and combining the Joint output of each pair of geophones of one spread with the joint output of a pair of geophones of each of the other spreads.

4. In seismic prospecting,. the method which comprises arranging a plurality of geophones as diflerent spreads with the horizontal spacing between the geophones of each spread corresponding with an odd number of half-wavelengths at a predetermined seismic frequency diflferent for the diflferent spreads, combining and filtering the outputs of each' pair of geophones to produce signals within a narrow band of frequencies including the frequency corresponding with the spacing of that pair and in which components due to horizontal waves are minimized because in phase opposition due to the spacing, and combining the signals of the corresponding pairs of geophones of the different spreads to produce a composite signal covering a wide band of seismic frequencies and corresponding with reflected waves to the substantial exclusion of horizontal waves.

CORNELIUS G. DAHM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,672,495 McCollum June 5, 1928 1,959,004 Owen May 15, 1934 2,087,702 Peters July 20, 1937 2,279,191 Adler Apr. 7, 1942 2,348,409 Parr May 9, 1944 Certificate of Correction Patent No. 2,473,469

June 14, 1949 CORNELIUS G. DAHM It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 3,.line 17, for the Word amplified read amplifier; column 4, line 62, after current insert voltage; column 6, line 71, for effefict read effect;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 31st day of January,

THOMAS F. MURPHY, 

